There have been conventionally known foams each obtained by causing a thermoplastic resin to pass through a die having plural delivery apertures therein to form foamed strands and then coalescing them together. For example, JP-A-1-502252 discloses a process of producing a foam structure, which comprises extruding a molten foamable thermoplastic composition through a die plate having an array of plural orifices or slits (delivery apertures) arranged such that adjacent strands or profiles extruded therethrough contact and coalesce to form a foam structure in which the strands or profiles are disposed substantially parallel to the longitudinal axis of the foam.
JP-A-2003-311806 discloses a method for making the delivery of a resin uniform in a width direction by arranging a multiple bifurcated manifold in an upstream section of an extrusion nozzle and providing the multiple bifurcated manifold with coat-hanger-shaped, flow rectification mechanisms at downstream ends thereof. Each of these coat-hanger-shaped, flow rectification mechanisms defines a resin flow path having a reduced cross-sectional area at a central part thereof and flared in a downward direction at opposite sides thereof such that a flow rate can be made uniform in a width direction of the thus-flared resin flow path.
In JP-A-1-502252, however, a thermoplastic resin needs to be extruded in the form of strands through the respective delivery apertures by an extruder when it is desired to produce a sheet-shaped or board-shaped foam having a relatively large width. As the product to be obtained is wide compared with a flow passage for the resin extruded from the extruder, the product is accompanied by a problem in that the product is thinner at opposite side regions thereof than at a central region thereof.
JP-A-2003-311806 describes to uniformly extrude a resin with a large width from a resin extruder by using a coat hanger die. However, an attempt to extrude a thermoplastic resin in the form of strands through the coat hanger die and the die plate disclosed in JP-A-1-502252 and having the array of delivery apertures involves a problem in that the product becomes thinner at its opposite side areas than at its central area, because the product to be obtained has a large width.
If a product has a smaller thickness at its opposite side areas than at its central area, the product involves not only a problem of inferior heat-insulating performance at such side areas but also a problem in the applicability to a building frame. These problems may be solved by trimming off the opposite side areas. This solution, however, is accompanied by problems. Described specifically, it takes time for the trimming and also for removing the trimmings so cut off, and therefore, is not preferred from standpoint of work efficiency. Unless the trimmed-off resin can be recycled, it has to be thrown away. These problems become particularly serious when the expansion ratio is 10 times or larger, because such a large expansion ratio tends to develop differences in the thickness of the resulting foam and hence to lead to wider cut-off trimmings.
On the other hand, JP-A1-2005-115720 discloses an extrusion die for extruding a thermoplastic resin. This extrusion die has coat-hanger-shaped spreading portions connected to respective downward outlets of a multiple bifurcated resin manifold, a resin holding portion arranged on a downstream side of these spreading portions, and a disturbed-flow-producing means arranged at the resin holding portion to reduce differences in the flow rate of the thermoplastic resin in a width direction and to make the flow rate uniform in the width direction. As examples of the disturbed-flow-producing means, a coil spring, a breaker equipped with a number of small holes, a rotor equipped with a number of shear blades or agitation blades, and a slit plate with a number of slits formed therein. It is considered desirable to arrange a means for making a flow rate uniform in a width direction because, when the flow of the resin from any one of the coat-hanger-shaped spreading portions is observed, the flow rate is not uniform in the width direction. Disturbance to the flows of the resin from the coat-hanger-shaped spreading portions is, however, not preferred for obtaining a uniform foam.